The stability of pharmaceutical products is of paramount importance to ensure safe and efficacious use for a sufficiently long time period. Unfortunately, the performance (safety, reliability, and efficacy) of most pharmaceutical products deteriorates over time. The causes of drug deterioration include chemical degradation (e.g., hydrolysis, oxidation, reduction and racemization), microbial contamination, and other mechanisms (e.g., precipitation).
Proteinaceous active ingredients are often of a labile nature and inherently instable. This leads to loss of biological activity during production, reconstitution and/or storage of protein-containing pharmaceutical compositions. These problems observed with proteins may be due to chemical instability, resulting in bond formation or cleavage (e.g., hydrolysis, oxidation, racemization, β-elimination and disulfide exchange), and/or due to physical instability of the second or higher-order structure of proteins without covalent bond-breaking modification (e.g., denaturation, adsorption to surfaces, and non-covalent self-aggregation).
Since degradation reactions are generally fastest in aqueous solutions and slowest in solid dosage forms, protein active ingredients are often formulated as lyophilized (i.e. freeze-dried) products. However, the lyophilized products have generally to be reconstituted with a pharmaceutically acceptable liquid (e.g., saline) prior to use. Therefore, lyophilized pharmaceutical products are considered less convenient than other dosage forms. Further, lyophilized products are usually more expensive and time-consuming to manufacture. Moreover, mismanagement can occur during the reconstitution process resulting in inaccurate dosing or sterility issues. All these disadvantageous can be overcome by the use of prefilled syringes. Therefore, prefilled syringes have been become increasingly popular as drug delivery devices.
However, if proteins are used as active ingredients, the limited stability of proteins rendered it often impossible for formulation scientists to use a prefilled syringe format. This applies especially to very dilute aqueous solutions of botulinum toxin (botulinum neurotoxin, BoNT). Such BoNT solutions are used in the treatment of a wide range of debilitating neuromuscular diseases (e.g., cervical dystonia, blepharospasm, spasticity, and hyperhidrosis) and in aesthetic medicine (e.g., treatment of facial wrinkles). There are seven homologous serotypes (A-G) of botulinum toxin, which are produced by different Clostridium spp., in particular C. botulinum, in the form of a complex consisting of a neurotoxic polypeptide and other (non-toxic) clostridial proteins (i.e. different hemagglutinins and a nontoxic, non-hemagglutinating protein). The neurotoxic polypeptide has a molecular weight of about 150 kDa and is activated by selective proteolytic cleavage to yield the active two-chain form consisting of a heavy chain (HC; includes the translocation domain and receptor-binding domain) and a light chain (LC; includes the catalytic domain) linked by a disulfide bond and non-covalent interactions.
Botulinum toxins are inherently instable and, in particular, are known to be highly unstable at alkaline pH and heat-labile. Additionally, it is known that dilution of the isolated toxin complex from milligram quantities to the much lower toxin concentrations used in solutions for injection (in the nanograms per milliliter range) presents significant difficulties because of the rapid loss of specific activity upon such great dilution.
Therefore, commercial preparations of botulinum toxin often come as vacuum-dried or lyophilized material. Examples include, for example, Botox® (onabotulinumtoxinA; Allergan, Inc.) and Dysport® (abobotulinumtoxinA; Ipsen Ltd.), which both contain the C. botulinum toxin complex of type A. Another example is Xeomin® (incobotulinumtoxin; Merz Pharma GmbH & Co. KGaA), which contains the pure neurotoxic component of serotype A (i.e. the neurotoxic polypeptide of a molecular weight of about 150 kDa) and is devoid of any other proteins of the Clostridium botulinum toxin complex (i.e. the different hemagglutinins and the nontoxic, non-hemagglutinating protein).
However, the lyophilized toxin products have a number of drawbacks including the need for reconstitution prior to use and concomitant sterility issues. In addition, the reconstituted toxin solution is often not entirely used in the clinical practice because not every patient and indication requires the same dosage. The unused amount of the reconstituted toxin solution can be stored at lower temperatures, but only for a short period of time. For example, after dilution with normal saline prior to use, Botox® and Dysport® are recommended to be used within 6 hours and 4 hours, respectively. Likewise, the package leaflet of Xeomin® specifies that after storage for more than 24 hours, the reconstituted Xeomin® solution shall no longer be used than for 24 hours and is then to be discarded.
To increase toxin stability, stabilizing proteins such as human serum albumin (HSA) are often added in the art. Other stabilizing strategies involve the use of non-proteinaceous stabilizing agents, for example surfactants, polyvinylpyrrolidone (PVP), disaccharides, polyols and the like. In addition, it was disclosed in WO 00/15245 that a liquid formulation of highly concentrated botulinum toxin type B (about 2500 U/ml) is stable for up to 30 months when stored in glass vials at 5° C. However, this prolonged stability requires buffering the pH of the formulation down to an acidic pH of between 5 and 6, which causes pain upon injection. Other known approaches to increase toxin stability rely on the addition of various non-protein excipients which, however, are unsuitable or undesirable for human use (see, e.g., WO 01/58472, WO 2006/005910, and WO 2007/041664).
Thus, there is still no injectable botulinum toxin presentation available which is not only stable over a long period to provide a sufficiently long shelf life, but is also convenient and easy to use, reduces medication errors, and minimizes the risk of contamination.